Retopo: Can we apply shrink-wrap to multires subdivisions?
After a few months I decided to revisit this and... I've got the answer!

It was right under my nose the whole time, right there in the interface - a button called 'Reshape'.

Reshape will... okay, I'll let the Blender Manual do the talking:


  Reshape - Copies vertex coordinates from another mesh. To use, first select a different mesh object with matching topology and vertex indexes,
  then Shift select the object you wish to copy vertex coordinates to
  and click Reshape.


So what's so exciting about this button and how does it further humanity's progress?

You can enjoy the flexibility of DynTopo and get great topology easily.

Here is the workflow: